Testosterone Cypionate (Test Cyp)

Testosterone cypionate is an oil-based injectable form of testosterone with the cypionate (cyclopentylpropionate) ester attached at the 17-beta hydroxyl position. Molecular weight: 412.61 Da. CAS number: 58-20-8. It was first synthesized in 1951 and FDA-approved in 1979 under the brand name Depo-Testosterone (manufactured by Pfizer).

This is the most commonly prescribed form of testosterone replacement in the United States, and for good reason. The cypionate ester gives it a half-life of about 8 days, which means stable blood levels can be maintained with weekly injections. Compare that to testosterone propionate (half-life 2-3 days, requiring injections every other day) or testosterone undecanoate (half-life 33 days, requiring injections only every 10-14 weeks). Cypionate sits in the sweet spot for convenience and stability.

The evidence base is enormous. The 2018 Endocrine Society Clinical Practice Guideline (PMID: 29562364) is the standard reference for TRT prescribing. It recommends testosterone therapy for men with symptomatic hypogonadism confirmed by two morning serum testosterone levels below 300 ng/dL. Symptoms include fatigue, low libido, erectile dysfunction, depressed mood, and loss of muscle mass.

A 2018 meta-analysis in Mayo Clinic Proceedings (PMID: 30236381) examined cardiovascular safety across 15 randomized controlled trials and found no increased cardiovascular risk with testosterone therapy. This was important because earlier observational studies had raised concerns. A separate 2016 study in the Journal of Clinical Endocrinology and Metabolism (PMID: 27355400) showed that testosterone treatment in obese hypogonadal men on a diet preserved lean mass and tripled fat loss compared to placebo.

Dosing protocols have evolved significantly. Traditional protocols used 200 mg every 2 weeks, but this creates a roller coaster pattern: testosterone peaks 2-3 days after injection, then gradually falls. Many men feel great the first week and lousy the second. Modern protocols favor smaller, more frequent doses. The most common current approach is 100-150 mg weekly, or even 50-75 mg every 3.5 days (twice weekly). These smaller doses produce much more stable blood levels and fewer side effects.

Subcutaneous injection is becoming the preferred route for many practitioners and patients. A 2014 study in the Journal of Clinical Endocrinology and Metabolism showed that subcutaneous testosterone cypionate produced equivalent serum levels to intramuscular injection with less injection site pain. The needles are smaller (27-29 gauge vs 22-25 gauge for IM), and patients can self-inject more easily.

Monitoring is essential on TRT. Standard blood panels include total and free testosterone, estradiol, hematocrit/hemoglobin, PSA, and lipid panel. Hematocrit above 54% requires dose reduction or therapeutic phlebotomy because elevated red blood cell counts increase blood viscosity and clotting risk. This is the most common clinically significant side effect of TRT.

Fertility preservation is a major consideration. Exogenous testosterone suppresses the HPG axis, reducing sperm production. Men who want to maintain fertility typically use HCG (250-500 IU 2-3 times weekly) alongside testosterone to keep intratesticular testosterone levels sufficient for spermatogenesis. Some practitioners use gonadorelin as an alternative to HCG for this purpose.

Testosterone cypionate is typically supplied in cottonseed oil at concentrations of 100 mg/mL or 200 mg/mL. Store at room temperature (20-25C). Protect from light. Vials that appear cloudy, discolored, or contain particulate matter should be discarded.